1. Field of the Invention
The present invention relates to techniques involving the filtering and processing of downhole sensor signals during measurement-while-drilling operations. More particularly, the invention relates to improved MWD data collection and data analysis techniques for determining information from downhole detectors mounted in a drill collar eccentrically rotating within a borehole.
2. Description of the Background
In the oil and gas drilling industry, data collected and plotted as a function of depth in a borehole is referred to as a well log. In recent years, increased emphasis has been placed upon downhole data collected and displayed while drilling. Measurement-while-drilling or MWD techniques are favored due to the ability of the drilling operator to alter procedures in real time, i.e., while drilling. Accordingly, a great deal of research has been and will continue to be expended to enhance MWD data sensing, transmission, filtering, processing and analysis techniques to better assist the drilling operator and improve drilling performance.
Those skilled in the downhole data collection and analysis art have recognized for years that most sensors on a drill collar do not accurately sense information if the drill collar is not centered, or the position of the logging tool not known, within the borehole due to the continually varying thickness of the drilling mud and its affect on the measured formation property. In recent years, substantially increased emphasis has been placed upon highly deviated boreholes and/or horizontal drilling, both of which increase the likelihood that the drill collar will not be centered within the borehole. When the bottom hole assembly (BHA) is substantially inclined rather than being vertical, the drill bit tends to form elliptical or slotted boreholes rather than boreholes with a circular cross-sectional configuration. Even if the borehole has a circular cross-sectional configuration, the bottom hole assembly (BHA) itself may not be centered in the borehole if the borehole diameter is substantially greater than the diameter of the bit. Although centralizers may be used to position the BHA closer to the geometric center of the borehole, such centralizers and/or stabilizers reduce the versatility of the BHA and thus are often not preferred when drilling highly deviated boreholes. Moreover, MWD sensors are typically located a distance of from 5 meters to 50 meters from the drill bit, so that the drill bit may in fact drill a borehole with a circular configuration, but the borehole geometry may change as "wash outs" occur. By the time the borehole has been drilled to the depth required for the sensors to monitor the previously drilled borehole and/or formation characteristics, the sensors are frequently no longer sensing information in a circular borehole. As an example of the problems involved in current MWD sensing and data analysis, nuclear radiation detectors are commonly mounted in a drill collar and lowered into a borehole to perform MWD well logging. As previously noted, it is often preferred that the rotating drill collar not be constrained by stabilizers or centralizers designed to maintain the BHA in the center of the borehole, and the drill collar is thus free to position itself in the borehole in a centered or concentric position, or may be touching a side of the borehole in a fully eccentric position. Of course there are numerous possible positions between the concentric and the fully eccentric positions, and frequently the position of the drill collar relative to the axis of the borehole is typically changing as the drill collar and drill bit are rotating. The position of the drill collar and the sensors relative to the axis of the borehole affects the signal produced by the nuclear radiation detectors, although the techniques of the prior art have not been able to practically correct for the changing eccentric rotation of the drill collar in the borehole.
In a recent paper by D. Best et al entitled "An Innovative Approach to Correct Density Measurements While Drilling for Hole Size Effects" 31st. SPAWLA, June 24-27, 1990 (paper G), a statistical approach was taken to interpret the rotating eccentered tool. The technique disclosed in this paper examines the average and standard deviation of the detector signal as the tool is rotated. The logging tool is calibrated in the fully eccentered position, and the assumption is made that the extreme value of the detector signal corresponds to a fully eccentered tool. This technique will not adequately handle the partially eccentered case, i.e., it will not compensate for the error of not being fully eccentered.
Techniques involving the measurement of formation density while drilling are described in an article by W. C. Paske et al entitled "Formation Density Logging While Drilling". This article generally describes an MWD logging technique currently offered by Sperry-Sun Drilling Services, Inc. in Houston, Tex. An advantage of this Sperry-Sun logging tool design is that the detectors, which may be Geiger-Mueler tubes, are placed around the circumference of the drill collar and are axially spaced so that signals are detected in all directions, thus increasing the signal-to-noise ratio and the useful information from the detectors. The placement of the detectors and shielding techniques enable some detectors to be very sensitive to the effects of centering (the front bank of detectors), while other detectors (the side and back banks of detectors) are less sensitive to eccentered rotation of the drill collar.
In recent years, increased emphasis has been placed upon techniques capable of correcting measurements taken in boreholes significantly larger than gage. In a recent article by W. C. Paske et al titled "Measurement of Hole Size While Drilling", a technique is described to determine the borehole size or caliper when using a rotating MWD logging tool. Using techniques described in this article, MWD generated formation density logs may be corrected to compensate for increased borehole diameters. An article by P. Wraight et al entitled "Combination Formation Density and Neutron Porosity Measurements While Drilling" discloses a measurement-while-drilling tool designed to provide accurate neutron porosity and density logs which are compensated by the mud standoff between the tool and the formation which occurs when the tool is rotating in an oversized borehole. These prior art techniques have increased somewhat the accuracy of well logs, but are not capable of accurately compensating for the affects of the MWD sensors mounted in a drill collar rotating eccentrically within a borehole.
The disadvantages of the prior art are overcome by the present invention, and an improved MWD data analysis technique is hereinafter disclosed for correcting the effects on MWD sensors within a drill collar eccentrically rotating within a borehole. The techniques of the present invention may be used to measure the amount of eccentering of the drill collar in the borehole and correct for the effects of eccentering. The present invention may also be used to measure the actual rotational speed of the drill collar within the borehole.